The only thing that really drives me nuts is the mandatory replacement of the Start Menu with the new "Metro" start page and the convoluted Control Panel/settings UI. If they'd just give the option to use the "classic" Start Menu and Control Panel I'd be ok with it.
I wouldn't mind the mandatory-ness if it was consistent. Some things are only accessible in the Metro settings, others only in the desktop settings, including ones that should be in both. Worse are the inconsistencies with the charms, copy and paste, and other aspects.
Windows 8 as a whole I think can be summed up well with the word "inconsistent" IMO.
There are fairly significant performance improvements. All the bad aspects of it can be ignored or changed (all of them are UI-related).
Eh? All improvements I've seen have been marginal at best, and it has performed worse in other categories.